We have continued studies on the characterization of the uridylyltransferase (UT) uridulyl-removing enzyme of Escherichia coli. An improved method for the purification of this enzyme has been worked out. We have corroborated the single subunit nature of this bifunctional enzyme by examination of this protein in a filtration column by HPLC. These studies have shown conclusively that the pure and partially purified UT.UR undergoes ready oligomerization to a dimer, trimer, and tetramer form possessing decreased activities. The amino acid composition of UT.UR has been determined.